Storage tube construction



April 12, 1960 D. w. DAVIS STORAGE TUBE CONSTRUCTION Filed Jan. 5, 1955WW I m gm

WWW qbq MW (mm //V VE N TOR. DEA N l4. DA V/S A T TORNE Y Un Stat i mmioSTORAGE TUBE CONSTRUCTION Dean W. Davis, Fort Wayne, Ind., assignor toInterna tion'aLTelephone and Telegraph Corporation Application January3,1955, Serial No. 479,329 6 Claims. (Cl. 315-12) .The present inventionrelates to storage tubes, and more particularly to a constructionwherein improved image reproduction is achieved.

A storage tube of the type contemplated by this invention is disclosedand claimed in Farnsworth Patent No. 2,228,388, granted January 1,.1941. The tube of this invention may take one of two forms, viz., asignal-to; image tube or a signal-to-signal tube. By signal-toimage ismeant that the tube will reproduce an" electrical signal in the form ofa visible image, whereas a signalto-signal tube is one wherein anelectrical image signal is reproduced in the form of an electricalsignal. From the following explanation ofthis invention, it will beevident that the principles may be adapted to either type of tube forachieving improved performance.

.It isan object of this invention to provide an image? amplifying tubewherein a material improvement in uniformity'of the reproduced image orsignal is achieved.

'It is another object of this invention to provide an image-amplifyingtube wherein a material improvement is achieved in the distribution ofbrightness over the area of the signal electrode, this signal electrodebeing either a phosphor screen or a metallic plate.

In accordance with the present invention, there is provided for use inan electron discharge device a perforate storage element having asurface area for providing an electrostatic charge image, a utilizationelectrode disposed adjacent to said-storage element and having anelectron-receiving surface which is juxtaposed with respect to saidstorage element surface area, and means included with said element andsaid electrode for equalizing electron current flow therebetween forachieving cross-sectional uniformity in the beam received by theutilization electrode. a

To the accomplishment of. the above and related objects, .myinvention-may be embodied inthe forms illustratedin the accompanyingdrawings, attention being called to the fact, however, that the drawingsare illustrativeonly, and that specific change may be made in thespecific constructions illustrated and described, so long as the scopeof the appended claims is not; violated.

In the drawings:

'Fig. 1v is a sectional of this invention; and

Fig. 2 is .3. firagmental sectional view of the storage screenthereof. Fg- Referring to Fig. 1, the cathode ray storage tube comprises anenvelope 1 having an enlarged cylindrical body portion '2. and a smallerdiameter angularly extending neck portion 3. Mounted in the end of theneck .3;is a beam-forming electron gun assembly 4 which generates a beamof electrons for covering an elemental area'only illustration of oneembodiment Patented 7 Apr. 12 1960 erally accomplished in-conventionalcathode ray tubes.

A perforate collector electrode 9of relatively large mesh size andtransmissivity is mounted adjacent the storage screen 5 on the electrongun side.

With suitable potentials applied to the electrodes thus far described, apositive charge image may be impressed on the storage screen 5, byimpacting writing beam electrons, is it is simultaneously being floodedby electrons from the gun 6, the flood gun being maintained at apotential slightly positive with respect to left-hand side of the screen5. The electrons flowing therefrom will not be attracted to the screenitself but instead will pass through the screen openings, to impinge theluminescent anode 7. As the flood electrons pass throughthe respectivescreen perforations, they are modulated in number in accordance with theelemental charges surrounding the corresponding perforations and will,therefore, impinge against the luminescent anode 7' with a densitydependout upon such modulation. By this means, the charge image on thescreen is optically reproduced on the anode 7. Y 7

Continuing with the detailed description, the screen 5, as seen in Fig.2, is comprised of a metallic screen 10 of, for example, 500 mesh size,one sideof this screen being covered by a dielectric material 11, suchas quartz. For exact details of construction of this screen and of the Iother tube structure, reference may be made to Farmsof. an image storagescreen generally indicated by the -5. A luminescent screen 7 of;conventional design I and worth application Serial No. 197,612, filedNovember 25, 1950, now Patent No. 2,754,449.

A magnetic focusing coil 12 of conventional design surrounds the neck 3for focusing the beam from the gun 4 onto the screen 5, and a suitablepair of deflection coils (only one coil 13 is shown) also surroundingthe neck 3 are mounted adjacent the focusing coil 12 for controllingbeam scansion of the screen 5. In this connection, any well-knownraster-forming scanning techniques may be utilized.

The gun assembly 4 is comprised of a cathode element 14, a controlelement 15, and an accelerating anode '16. The flood gun assembly 6 iscomprisedof a cathode =17 and an accelerating anode 18, these'partsbeing so arranged as to direct the electron flow from the cathode17 overthe entire front face of the screen 5.

Input video frequency signals derived from a suitable video circuitrepresented by the block 19 may be applied through a condenser 20 to thecontrol grid15. A negative biasing battery 21 is connected through aresistor 22 to the control grid 15, and a cathode biasing battery 23 isconnected between ground and the cathode 14 with the minus terminal ofthe battery, 23 being connected to the positive terminal of the battery21. Anode 16 voltage is supplied by battery 37 having a negativeterminal which is connected to thenegative terminalof battery 23. Aconventional scanning wave generator capable of generating a current ofsawtoothwave form is conductively connected to the deflection coil 13for controlling the scanning mov'ementof the electron beam.

The luminescent anode 7 is supplied with a relatively high potentialfrom a battery 26. Another battery 27 has its positive terminalconnected to the metal backing. 10 of the storage screen 5 (Fig. 2);However, the potential of the front (insulating) side] of the storagescreen is the moreimportantfactorin .the operation of the tube than isthat of the metal backing, and it has been found that, under someconditions, the polarity of battery 27 may be reversed. Thecollectorelectrode'9-"has a positive potential applied from the battery 28. TheConductive coating 8 inside the tub'eis provided witha'positivepotential from battery 29. The anode 18 is positive by connection tobattery 36. The cathode 17 is connected through a resistor 31 to.groundwhich is common to all of the negative terminals of thelast-mentioned batteries. Cathode 17 is further coupled through acapacitor 32 to an -erase pulse generator 33, which is of conventionaldesign, for'supplying a square wave pulse. This erase pulse generator 33maybe a one-shot multivibrator which is triggered into single cycleoperation by a wave derived from the aforementioned scanning wavegenerator.

In operation, the beamof electrons emitted by the electron gun 4 isscanned over the screen 5 in the usual raster forming manner by means ofthe deflection coils. The electrons in this beam are caused -to travelatsuch velocity as to produce greater than unity secondary emission onthe impinged elemental areas of the screen insulator 11, so that'as thebeam is scanned over the area of the screen, a charge pattern ofpositive potential is formed. The low velocity electrons from the floodgun 6 are attracted toward the luminescent anode 7 by reason of therelatively high potential applied thereto, these flood electrons passingthrough the mesh openings of the screen 5 before striking the screen 7.The number of flood electrons passing through the individual meshopenings are modulated in accordance with the charge pattern on thescreen, the modulated electron flow there- Ser. No. 362,473, filed June.18, 1953. Insofar as is necessary for a complete understanding of thisinvention, this Farnsworth et al. application is made a part hereof.

Considering for the moment that the anode 7 is a phosphor screen, it hasbeen noted in prior art devices that obectionable shading over the areasthereof appeared. For example, in certain instances the central portionof the screen 7 would be unnaturally brighter than the outer regions. Inovercoming this objectionable operation, the spacing between the storagescreen 5 and the phosphor anode 7 is varied over the lateral extentthereof as is clearly illustrated in Fig. 1. Having a flat screen 5, ithas been found that the phosphor screen 7 should be convexlycurved-outwardly to vary the aforementioned spacing. The exact degree ofvariation in this spacing, or stated in other words, the exact degree ofcurvature of the phosphor anode 7 will vary according to the differentsizes and characteristics of the finby representing the charge patternon the screen 5 and serving to excitethe display of an equivalent imageby the screen 7.

During reproduction of the image on screen 7, the potential relationshipbetween the cathode 17 and storage element Sis such that the electronsreach element 5 with a low velocity. This velocity is adjusted to besuch as will not cause impingement of the flood electrons on the chargedscreen 5 but instead will merely form a cloud of electrons (or virtualcathode) adjacent the charged face of the screen 5. The electrostaticfield produced by the anode 7 may be considered to reac through theapertures of the screen 5 and draw the flood electrons therethrough. Ithas been'found that in order to achieve suitable flood electronvelocity, the cathode should be preponderantly positive; meaning thatfor the most part the elemental image charges on the screen 5 are at anegative potential with respect to the cathode 17. Further, the metallicbacking 10 of screen 5 may be operated slightly positive with respect tocathode 17 at a value which has been found should not exceed a fewvolts. However, the exact potential relationship b'etween'cathode 17 andscreen 5 will depend upon tube operating requirements.

An image pattern is formed on the screen 5 at the completion of theeffective vertical scanning cycle ap- 'plied to the deflection coil 13.Since it is desired to erase this charge image prior to the nextpicture-forming scansion, the retrace portion of the scanning wave isutilized to initiate the erasing pulses 34 in synchronism therewith.These erasure'pulses 34 are made sufficiently negative so that adiflerence of potential between'the cathode 17 and the charge pattern onthe storage screen 5 will be set up which substantially increases thevelocity of the electrons emitted from the cathode 17 and causes themto-impinge upon the charged areas of .the screen. These electrons supplythe deficiency of electrons as represented by the positive chargepattern and thereby establish the potential of the screen at a uniformnegative value corresponding to the potential of the pulse 34.

This method of pulsing the hood gun for alternately providing erasingand reading? electrons is disclosed and claimed in 'Farnsworth et a1.application ished tube. However, this curvature may be determined inaccordance with the following teachings.

By curving the phosphor anode 7 into the proper shape, the objectionableshading mentioned hereinabove is eliminated, with the consequence. thatthe brightness characteristic over the entire lateral extent of theanode 7 is made uniform.

This elimination of shading is believed to occur for the followingstated theoretical reasons. An electrical field exists between thescreen 5 and anode 7 which tends to penetrate the openings in the screen5. These penetrating fields (see the equi-potential lines 35 of Fig. 2)accelerate flood electrons through the respective screen openings towardthe anode 7. However, difiiculty arises due to the fact that althoughthe velocity of flood electrons in the axial region of the tube may bethe same as the velocity for the outer or peripheral regions, the anglewhich the velocity vector makes with the normal to the plane of theinsulator is greater in the outer regions It is the normally-directedvelocity component which determines whether a flood electron will beable to pass through the minimum potential existing in the insulatormesh holes or whether it will be deflected away from the insulatortoward collector electrode 9. Therefore, for the same minimum potentialin the mesh holes of the two regions, more low velocity electronsnormally are attracted to the central region of the anode 7 than for theouter regions, whereupon the central region is unnaturally brighter.

The effects of this difierential in electron normal velocity areovercome by increasing field penetration from the anode 7 through theopenings of the screen 5 for the outer portions only of the latter.Stated conversely, the accelerating electrical field produced by the.anode 7, which reaches through the openings of the screen 5, is maderelatively less in the axial region of the screen 5 than for the outerregions. This is achieved by spacing the central portion of the anode 7farther away from the central portion of the screen 5 than the outerportions, such that a more positive minimum potential is produced in theouter regions between the two elements 5 and 7 than for the centralregion. The obliquely-directed low velocity flood electrons existing inthe outer regions thereby penetrate electrode 5 to the same degree asthe normally-directed electrons in the axial region, and by spacing thescreen 5 and anode 7 the proper distances apart, an equal number ofelectrons will reach the anode 7 in all regions. Thus, electron currentflow over the lateral extent of the anode 7 is equalized or madeuniform, thereby eliminating the objectionable condition of shading.

In some instances, where the flood gun is eccentrically positioned withrespect to the storage screen 5, it will be necessary to shape the anode7 such as to counteract the shading effect produced by the unequalnormal electron velocities. 'It will now be apparent that the particular"shape of the'anode'7 will vary depending upon the position of the floodgun 6 or any other low velocity electron gun used to scan the storagescreen 5. This shape is always made such as will produce equal electrondensities over the lateral extent of the anode 7 as evidenced by"uniform, natural distribution of luminescence or brightness. 7

The basic concept of this invention, as will now be apparent, resides incorrecting for the shading characteristics of thetube, and while onemethod of achieving this correction is specifically described, othermethods will appear as obvious to a person skilledin the art. Forexample, this correction is achieved by varying the sizes of theopenings of the screen iov er the lateral extent thereof'such that forthe areas thereof. coinciding with low density electron flow, electrontransmission through the screen will be increased. This greatertransmission also allows greater field penetration from the anode 7, thenet result of the increased transmission and penetration being theequalizationof electron currents arriving at the anode 7.

What is claimed is:

. 1. For use in an electron discharge device, a perforate storageelement having an extended bi-dimensional surface area treated forproviding an electrostatic charge image responsive to bombardment withhigh velocity electrons, a source of low velocity electrons on one sideof said element arranged to flood said surface area thereof, autilization element disposed adjacent the other side of said storageelement and having an electron-receiving surface which is coextensivewith said other side of said storage element for receiving said lowvelocity electrons which pass through said storage element, saidelectron source having an area which is small with respect to the areaof said storage element so that said storage element normally tends topass a disproportionate electron current in one regional area, one ofsaid elements having its surface curved with respect to the surface ofthe other element with the distance defined therebetween varying overthe surfaces thereof in accordance with the aforesaid disproportionateelectron current whereby the electron current passed through saidstorage element is substantially uniform over the entire surface areathereof.

2. For use in an electron discharge device, two bidimensional extendedside-by-side electrodes spaced apart and having juxtaposed surfaceareas, means for applying a control potential between said electrodeswhich potential produces an electrical field extending from oneelectrode to the other, a cathode arranged to flood one of saidelectrodes with low velocity electrons, saidone electrode beingperforate to pass said electrons received from said cathode, said oneelectrode having means for providing and storing an electrostatic chargeimage on one surface area thereof responsive to bombardment of saidsurface area with high velocity electrons, saidcathode having a smallarea with respect to the area of said electrodes so that said oneelectrode normally tends to provide a disproportionately greaterelectron current in one elemental area, one of said electrodes havingits surface area curved with respect to the surface area of the otherelectrode with the distance defined therebetween being greater adjacentsaid elemental area than for the remain ing portions of said surfaceareas whereby said electrical field provides a uniform cross-sectionalflow of electron current between said electrodes over said surfaceareas.

3. For use in an electron discharge device, two elements spaced apartand having juxtaposed surface areas, means for applying a controlpotential between said elements which potential produces an electricalfield extending from one element to the other, said one element beingflat and perforate and including a surface layer for receiving andstoring an electrostatic charge image,

a first cathode for bombarding said surface area of said one elementwith high velocity electrons to impress said electrostatic charge imagethereon, a second cathode'for covering the chargedarea of said oneelement with low velocity electrons which pass therethrough to bereceived by the other element, said second cathode having a small areawith respect to the area of said elements whereby an unequal electroncurrent flow normally tends to pass through said one element over thelateral extent thereof, said other element having its surface curvedoutwardly with respect to the surface of said one element with thedistance defined therebetween varying over the surface thereof inaccordance with said unequal current flow thereby to equalize thecurrent flow through said oneelement over the lateral extent thereof.

4. An electron discharge device comprising a bi-dirnensional foraminousgrid of substantial area having a secondary-electron emissivecharge-storage surface on one side thereof, a point-source of lowvelocity electrons spaced from and adapted to bombard the entire area ofsaid one side, a phosphor plate spaced from and coextensive with theother side of said grid and adapted to receive electrons moving axiallythrough the perforations of said grid, said plate being concaved withrespect to said grid so thatthe spacing between said grid and plate isprogressively less at points removed from the shortest electron-sourceto grid line.

5. An electron discharge device comprising a pointsource of low velocityelectrons, a planar-type foraminous grid disposed to be bombardedthroughout :one side by said source, aj-planar-type anode electrodecoextensive. with and spaced from the other side of said grid andadapted to receive electrons through the perforations of the grid, saidgrid and anode electrode being relatively concaved to producegrid-to-anode spacing and voltage gradiants at any point on the gridproportional to the 'obliqueness of the line from said point to saidelectron source.

6. In a storage tube: a planar storage screen having a conductivesurface and a dielectric surface; an electron gun spaced from saiddielectric "surface for directing a high velocity electron beam towardsaid dielectric surface; means for scanning said dielectric surface withsaid high velocity electron beam thereby impressing a positive chargepattern on said storage screen; a secondary electron emission collectorscreen positioned between said dielectric surface and said electron gun;an anode spaced from said conductive surface; means for establishing anelectrical field between said anode and said conductive surface; anotherelectron gun spaced from said dielectric surface for directing lowvelocity flood electrons toward said dielectric surface and through theopenings in said storage screen onto said anode under the influence ofsaid electrical field whereby said electrons are modulated in accordancewith the charge pattern on said storage screen; said anode being concavewith respect to said storage screen to provide field strengthsincreasingly greater toward the outer edge of said anode therebyproviding uniform electron flow from said conductive surface to saidanode throughout the lateral extent thereof. 7

References Cited in the file of this patent UNITED STATES PATENTS2,532,339 Schlesinger Dec. 5, 1950 2,547,638 Gardner Apr. 3, 19512,689,314 Gunderson Sept. 14, 1954 2,806,174 Pensak Sept. 10, 1957

